Polar bears eat seals. That single fact defines nearly everything about the species, from where it lives to the shape of its skull to the chemistry of its blood. Ringed seals make up roughly half the diet in well-studied populations, with bearded seals, seabirds, and scavenged whale carcasses filling out most of the rest. But the full story of what polar bears eat, and what happens when their preferred menu becomes harder to find, reaches into genetics, seasonal fasting, cub-rearing, pollution, and the increasingly uncertain future of Arctic sea ice.
Ringed Seals and the Rest of the Marine Menu
Fatty acid analysis of polar bear fat tissue in the southern Beaufort Sea found that ringed seals accounted for about 46% of the diet, followed by bearded seals at roughly 20%, seabirds at 17%, bowhead whale at 15%, and beluga whale at just 2%.1PubMed Central. Fatty acid‐based diet estimates suggest ringed seal remain the main prey of southern Beaufort Sea polar bears despite recent use of onshore food resources Those proportions shift from region to region. In the Canadian Beaufort Sea, most of the variation in what individual bears ate was explained by longitude, which reflects where different prey species are available, along with local sea ice conditions.2PubMed. Drivers and consequences of apex predator diet composition in the Canadian Beaufort Sea
Ringed seals matter so much because they are the most abundant seal in the Arctic and spend much of their lives on or near sea ice, making them accessible to bears that hunt from ice platforms. A polar bear’s classic hunting technique is to wait motionless beside a breathing hole in the ice until a seal surfaces, then strike with its forepaws. Bears also stalk seals resting on the ice surface, creeping forward over dozens of meters before charging the last few body lengths. Both strategies depend entirely on the presence of sea ice.
How Size and Sex Influence What Bears Eat
Not every polar bear eats the same menu. Body size creates a clear divide. Large bearded seals and walruses are consumed most often by older, larger males, while ringed seals and harbor seals are more important to younger bears and females. Older males also show the greatest dietary diversity, apparently because their size lets them overpower larger prey.3Ecological Monographs. Polar bear diets and Arctic marine food webs: insights from fatty acid analysis A 500-kilogram adult male can tackle a bearded seal that weighs over 250 kilograms, while a 200-kilogram subadult would struggle with that same animal and sticks to smaller, easier kills.
This size-based dietary split matters because it affects how vulnerable different segments of the population are when sea ice declines. Smaller bears that depend on ringed seals may lose access sooner if ice retreats early in the season, while large males with a broader prey base might cope a bit longer. That does not mean the big bears are safe; it means the dietary pressure hits unevenly.
Built for Blubber at the Genetic Level
Polar bears eat a diet that would be catastrophic for a human heart. Seal blubber is extraordinarily rich in fat, and bears consume it preferentially, sometimes leaving the lean meat behind. Genomic analysis has revealed how they get away with it. Researchers comparing polar bear and brown bear genomes found that the strongest signals of positive selection in polar bears fell on genes tied to cardiovascular function. Nine of the top 16 genes under the strongest selection are associated with heart and vascular disease in other mammals.4PubMed Central. Population genomics reveal recent speciation and rapid evolutionary adaptation in polar bears
The standout gene is APOB, which encodes the main protein component of LDL cholesterol, the kind cardiologists warn people about. Mutations in APOB appear to help polar bears clear cholesterol from their blood efficiently, preventing the artery-clogging buildup that the same diet would cause in humans.5Cell. Genome Sequencing of the Polar Bear Identifies Rapid Adaptation to a Extreme and High-Fat Diet In essence, polar bears evolved a cardiovascular system rebuilt from the inside to handle lifelong elevated blood fat levels. This adaptation is not just impressive biology; it is direct evidence that their extreme fat-based diet is not a quirk of opportunity but a deep evolutionary commitment.
A Skull Redesigned for Seal Flesh
The evolutionary commitment to seal meat shows up in the skeleton, too. Polar bears are the only living bear species with a fully carnivorous diet, and their skulls reflect it. Compared to brown bears, polar bear skulls are narrower, with a more elongated shape suited to catching and consuming soft-bodied marine prey. But this specialization comes with a trade-off: biomechanical modeling shows the polar bear skull is weaker than a brown bear’s and less capable of processing the tough, fibrous foods that omnivorous bears handle easily.6PLOS ONE. Biomechanical Consequences of Rapid Evolution in the Polar Bear Lineage
This finding undercuts the hopeful idea that polar bears could simply switch to a plant-heavy or omnivorous diet if seals become harder to find. Their jaws are literally not built for it. Brown bears can crack open nuts, grind fibrous roots, and chew tough vegetation because they retained the robust skull of a generalist feeder. Polar bears traded that versatility for efficiency at the one thing they do best: peeling blubber off a seal carcass.
Feast, Fast, and the Spring Feeding Frenzy
Polar bear eating is not spread evenly across the year. Spring is the critical feeding window. Sea ice conditions are ideal, ringed seal pups are born in snow lairs on the ice, and bears gorge during a period sometimes called spring hyperphagia. The energy packed away during these months has to carry bears through the leaner stretches that follow. Research tracking energy balance during spring found dramatic variation among individual bears: some gained weight rapidly, while others actually lost mass, often because mating behavior cut into feeding time.7Arctic Science. Variation in energetic balance among free-ranging polar bears during the spring mating and foraging season
When food runs out, polar bears are remarkably good at not eating. They can enter a fasting state at any time of year, not only during denning, and manage their body chemistry to stretch fat reserves. Researchers studying the ratio of certain waste products in their blood found that polar bears rapidly adopt physiological mechanisms to recycle nitrogen and conserve energy, making them among the most proficient fasting mammals on land.8Canadian Journal of Zoology. Seasonal changes in the ratio of serum urea to creatinine in feeding and fasting polar bears One elegant byproduct of burning fat is that the breakdown produces water, reducing the bear’s need to drink. The end products of fat combustion are carbon dioxide (exhaled) and metabolic water (used by the body), so a fasting bear on stored blubber is essentially drinking its own fat reserves.9Mayo Clinic Proceedings. Black Bears and Polar Bears
Energy expenditure while on land varies enormously. A study tracking bears with GPS collars and metabolic sensors during the ice-free season in western Hudson Bay found that daily energy expenditure varied more than fivefold among individuals. Subadult females burned the most energy per kilogram of body weight, while adult males, being larger and moving less, burned the least.10PubMed Central. Polar bear energetic and behavioral strategies on land with implications for surviving the ice-free period Pregnant females fell somewhere in between, conserving energy for the denning season ahead.
What Polar Bears Eat When There Are No Seals
As sea ice shrinks and the ice-free season lengthens, polar bears increasingly turn up on land doing things that would have seemed unusual a few decades ago: eating berries, raiding bird nests, and even chasing reindeer. How much these foods actually help is one of the most debated questions in polar bear research right now.
In western Hudson Bay, scat analysis shows that bears on land eat berries, particularly alpine bilberry and black crowberry, more often than other terrestrial foods, especially in late summer and early fall when the fruit ripens.11PubMed Central. Dietary composition and spatial patterns of polar bear foraging on land in western Hudson Bay Bears have also been documented raiding colonial seabird nests. In surveys of eider duck colonies along more than 1,000 kilometers of Canadian Arctic coastline during record-low ice years, researchers found bear activity at over a third of the colonies and estimated that bears destroyed more eggs than traditional nest predators like foxes and gulls.12PubMed Central. Longer ice-free seasons increase the risk of nest depredation by polar bears for colonial breeding birds in the Canadian Arctic
On Svalbard, researchers recently documented the full course of a polar bear hunting and killing an adult reindeer, and reported several other recent bear-reindeer encounters, suggesting that this predation is becoming more common.13Polar Biology. Yes, they can: polar bears Ursus maritimus successfully hunt Svalbard reindeer Rangifer tarandus platyrhynchus The footage was striking: the bear pursued a reindeer into the sea and drowned it, using the water as a hunting environment, a strategy that draws on swimming skills honed for ice-edge seal hunting.
Despite these observations, the weight of the evidence so far is sobering. Only small numbers of polar bears have been documented eating terrestrial foods in meaningful quantities, and the calories consumed on land have been insufficient to offset the energy lost when ice-based hunting disappears early.14Frontiers in Ecology and the Environment. Can polar bears use terrestrial foods to offset lost ice‐based hunting opportunities? A modeling study in western Hudson Bay found that, theoretically, the total available calories from eggs, geese, and caribou could far exceed what the most vulnerable segment of the population (starving adult males) would need to survive the ice-free period. Available calories from caribou alone could exceed needs by a factor of several hundred.15PLOS ONE. The Energetic Value of Land-Based Foods in Western Hudson Bay and Their Potential to Alleviate Energy Deficits of Starving Adult Male Polar Bears The problem is that theoretical availability and actual consumption are very different things. Polar bears are not efficient caribou hunters, they are rarely present in the right places at the right times, and competition with other predators and scavengers reduces what is actually accessible.
Whale Carcasses as a Communal Resource
One terrestrial food source does deliver significant energy: whale carcasses. When a bowhead or gray whale washes ashore or is killed by subsistence hunters and the remains are left accessible, polar bears gather in numbers that can reach several dozen. Research in the southern Beaufort Sea found that subsistence-caught whale carcasses provided an upper threshold of more than 4,000 seal equivalents of energy, potentially enough to meet the average consumption needs of about 80% of the local bear subpopulation during autumn and winter.16Polar Biology. Significance of autumn and winter food consumption for reproduction by Southern Beaufort Sea polar bears, Ursus maritimus
These feeding events are genuinely important for individual bears, especially in years when fall ice forms late. But they are unpredictable in timing and location, and the energy subsidy depends heavily on human hunting practices. A whale carcass is a windfall, not a strategy that scales reliably across the entire species.
Caching Kills and Defending Meals
Polar bears do not always eat everything at once. Nineteen documented observations of short-term food caching between 1973 and 2018 across Svalbard, Greenland, and Canada show that bears sometimes cover uneaten portions of a kill with snow to hide it from other bears and scavengers. This behavior is most common with larger kills: a ringed seal pup is typically devoured entirely in one sitting, but an adult ringed seal, harp seal, or bearded seal may leave enough leftovers to be worth burying. The caching bear usually stays nearby, resting and returning to feed again later.17Arctic Science. The ecological and behavioral significance of short-term food caching in polar bears (Ursus maritimus) This is not the deep, long-term food storage seen in some other predators; it is more like throwing a napkin over your plate to keep someone else from grabbing it while you take a nap.
What Mothers Feed Their Cubs
Polar bear cubs are born in winter dens after months of maternal fasting and survive entirely on their mother’s milk before they can follow her onto the sea ice. That milk is extraordinary. Fat content averages around 36% for mothers with newborn cubs emerging from dens in spring, declining to about 28% the following autumn and dropping further to around 21% by the time the cubs are yearlings.18Canadian Journal of Zoology. Aspects of milk composition and lactation in polar bears Gross energy in the milk follows the same decline, falling from about 16.9 kilojoules per gram for three-month-old cubs to 9.7 kilojoules per gram when the offspring are nearly three years old. The pattern makes biological sense: very young cubs need the highest caloric concentration because they are growing fast and have no other food source, while older cubs increasingly supplement their diet with seal kills.
The fatty acid profile of the milk is also distinctive. Roughly three-quarters of the fat is unsaturated, split between monounsaturated and polyunsaturated fatty acids, with saturated fat making up less than a quarter.19PubMed. Milk composition in free-ranging polar bears (Ursus maritimus) as a model for captive rearing milk formula This composition closely mirrors the fatty acid profile of seal blubber, which is the mother’s primary fuel source. In zoos, getting the milk formula right for orphaned cubs has been a persistent challenge, and wild polar bear milk composition data are used as the benchmark.
Contaminants That Travel Through the Food Chain
Being an apex predator at the top of a marine food chain comes with a toxic cost. Persistent organic pollutants, particularly certain polychlorinated biphenyls (PCBs) and pesticide residues like oxychlordane, accumulate at each step of the food web. A seal absorbs more than the fish it ate, and a bear absorbs more than the seal. Research on Alaskan polar bears found strong relationships between contaminant levels and dietary position: bears feeding at higher trophic levels had higher concentrations of the most resistant PCBs, with the statistical models explaining up to 93% of the variation in certain contaminant levels in female bears.20Canadian Journal of Zoology. Dietary biomagnification of organochlorine contaminants in Alaskan polar bears Males and females differed because of their different diets: larger males eating larger, higher-trophic-level prey accumulated contaminants differently than females eating mostly ringed seals.
These pollutants can affect immune function, reproduction, and hormone regulation. For a species already under pressure from habitat loss, carrying a body burden of industrial chemicals adds an invisible layer of stress that is difficult to study in wild populations but clearly present in tissue samples.
Garbage in Bear Country
An emerging and less-discussed dietary threat is garbage. Analysis of stomach contents from 42 polar bears on Alaska’s North Slope between 2010 and 2020 found user plastics in nearly 29% of the animals and other rubbish in about 12%. A third of the bears showed signs of acute stomach inflammation, and half of those gastritis cases occurred in animals with plastics in their stomachs.21Ursus. Anthropogenic waste ingestion of Southern Beaufort Sea polar bears, Alaska (2010–2020) As bears spend more time near human settlements during long ice-free periods, garbage dumps and stored food become attractants. Managing waste in Arctic communities is not just a local sanitation issue; it is a wildlife health issue with direct physiological consequences for bears.
How Diet Is Reshaping the Gut
Dietary changes do not just affect body condition; they may be rewriting the microbial communities inside polar bears. A comparison of gut microbiomes between the southern Beaufort Sea subpopulation, where sea ice loss has pushed bears toward more varied and terrestrial foods, and the east Greenland subpopulation, which retains more traditional ice-based hunting, found substantial differences. Associations between the shifting Beaufort Sea microbiome and diet suggest that the foraging changes tied to ice loss are altering gut bacteria, with potential downstream effects on nutrition and physiology.22Scientific Reports. Distinct gut microbiomes in two polar bear subpopulations inhabiting different sea ice ecoregions Whether those microbiome shifts help bears extract more energy from novel foods or simply reflect a degraded version of the seal-tuned original is still an open question. It is the kind of finding that sounds abstract but could turn out to be one of the early warning signals of population-level dietary stress.